Self-contained diving systems are well-known in the art and systems are well-developed which permit diving to depths of approximately 500 meters. Self-contained systems fall into two general categories, air diving in which compressed air is used as breathing gas and secondly, mixed gas diving in which the diver is supplied with one or more artificial mixtures of gases, suitable for the depth and phase of the operation.
Traditionally, this split between air and mixed gas diving has taken place at depths of 50 meters (165 feet). For diving operations to less than 50 meters compressed air would normally be used, while for depths of greater than 50 meters mixtures of helium and oxygen would typically be used.
While air is a satisfactory breathing gas on or near the surface, it has serious limitations as a diving gas. As a diver proceeds below 50 meters, the increasing ambient pressure progressively renders air unbreathable. This condition results from two causes: nitrogen, which constitutes approximately 79% of air, becomes narcotic as ambient pressure increases; and oxygen, which constitutes approximately 21% of air, becomes toxic under the same conditions. To overcome these effects, the diver is fed artificial breathing mixes consisting of helium and oxygen, helium/nitrogen and oxygen, hydrogen/helium oxygen, neon and oxygen or exotic mixtures of deuterium and oxygen. When mixed in the correct proportions such breathing mixtures enable diving operations to be carried out at considerable depth. The maximum depth of such operations has not yet been determined. However, one limiting factor for a self-contained system is the large volume of breathing gas required. As the diver descends, gas consumption increases rapidly and is determined by the following expression; gas usage at a given depth per minute equals gas usage at surface for the given work load, multiplied by the absolute pressure. Additionally, even a short duration dive at depth requires an extended de-compression time. For example, a dive to 160 meters for only 15 minutes requires approximately seven hours of decompression. Although a diver in this example may ascend rapidly to approximately 40 meters, he must spend approximately six more hours ascending from 40 meters to the surface. Typically, these long decompression times allow a brief duration dive using a self-contained system to approximately 200 meters as a practical limit due to the volume of breathing mix which must be carried even with closed circuit equipment.
From the foregoing discussion, it can be seen that the diver's breathing mixture must meet certain criteria. The diluent gas should be relatively inert and have no appreciable narcotic or other detrimental effect. The breathing mixture must have adequate oxygen content to support life but not so great content as to produce toxicity and must be supplied at a suitable pressure and temperature. The critical factor in controlling the oxygen content is the partial pressure of constituent oxygen (ppO.sub.2).
Partial pressure of oxygen in a particular mixture is the pressure oxygen alone would have if the other gases were removed from a fixed volume of mixture. The physiological effect of oxygen depends upon its partial pressure in a mix, becoming increasingly toxic as the partial pressure increases above the normal level found in air at sea level. Typically at sea level, the partial pressure of the oxygen constituent of air being 0.21 bar.
There are two major expressions of oxygen poisoning, one which affects the central nervous system (CNS) and the other which affects the lungs. CNS poisoning becomes a significant factor as the partial pressure of oxygen approaches 2.0 bar. It gives rise to various symptoms, the most serious of which is a convulsive seizure, similar to an epileptic fit. These seizures last for about two minutes, and are followed by a period of unconsciousness. The diver will regain consciousness after some 15 minutes to repeat the symptoms if the oxygen pressure remains unchanged. The obvious danger to a diver is the loss of control while in the diving environment and the resultant danger of drowning. The point at which an individual diver will be affected by CNS oxygen poisoning varies widely and is also significantly affected by workload. As such, various companies and navies have laid down guidelines for the maximum permissible oxygen partial pressures under various circumstances. Typically, values between 0.8 bar and 1.8 bar are used for diving operations and 0.3 bar to 0.5 bar for storage while in saturation. Storage while in saturation refers to operation wherein divers are recovered from depth in a closed and pressurized diving bell and then transferred under pressure to a chamber complex, typically onboard ship. Saturation refers to a technique employed for deep commercial diving operations. As discussed previously, as time at depth increases so the time necessary to decompress increases. However, a state is reached after which further increases in bottom time do not further increase the time to decompress. This state is referred to as saturation. Typically divers are stored at pressure in the chamber complex for several days or weeks, transferred to a bell for work periods and then lowered to the sea floor. At the end of the period, the system is then slowly decompressed over a period of several days or weeks, depending on the depth at which the system was operating.
Pulmonary oxygen poisoning, on the other hand, results from prolonged exposure to oxygen partial pressures above 0.5 bar, and causes irritation and damage to the lungs. The onset of this form of poisoning is insidious and progressive, and is not as dramatic as CNS oxygen poisoning. It will be apparent from the foregoing that the ppO.sub.2 in a breathing mix should be kept to less than 1.8 bar and above 0.2 bar. This range is quite wide and there are, optimum values appropriate to different circumstances as discussed further hereafter.
Phases of decompression can create a preferential diffusion gradient for the elimination of the inert gas load. Typically for the deep dive, the descent and bottom time would be completed on helium/oxygen mixes. During the course of the decompression, an inert gas change to neon or possibly nitrogen would be made, which would have the effect of speeding-up the elimination of the helium absorbed at depth while limiting the absorption of the second applied inert gas.
Additionally, current closed circuit systems off-gas both oxygen and diluent gas. This off-gassing typically occurs on descent where the pre-set pressure of oxygen is greater than the pressure in the initial stages of the dive. For example, a pre-set pressure of 1.8 bar may be used during descent to limit the up-take of inert gas. With this setting, oxygen off-gassing occurs from the surface down to approximately 8 meters. Likewise during ascent, off-gassing also causes excessive use of the breathing mix gases.
The net effect of these limitations is that open circuit compressed air diving is limited to approximately 50 meters in depth and similarly open circuit mixed gas diving is limited to approximately 100 meters in depth. Also closed circuit mixed gas diving is also currently limited to approximately 100 meters due to the problems of off-gassing and less than optimal oxygen set point. Compressed air diving is limited because the oxygen partial pressure is too low during the initial descent, thereby causing a greater absorption of nitrogen. Thereafter, the partial pressure of oxygen is too high causing oxygen poisoning and, because of the high absorption of nitrogen, the possibility of nitrogen narcosis. Finally, the partial pressure of oxygen is too low on ascent causing an extended decompression time. Likewise, the lack of control of oxygen partial pressure in self contained mixed gas diving limits the practical depth. First, the off-gassing of oxygen during the initial part of the dive reduces the available oxygen and then the lack of partial pressure control extends the decompression time on ascent. Finally, off-gassing again occurs during the final ascent. The problems of off-gassing and sub-optimal oxygen partial pressure control limit the effective depth of self contained diving systems to approximately 100 meters by the inability to carry sufficient breathing mix to meet the required time for decompression.